1. Field of the Invention
The present invention relates to the detection of electrical currents and more particularly to a device for detecting low magnitude electrical currents.
2. The Prior Art
There are a number of situations where it is necessary to detect electrical currents so that they can be monitored, and there have been a number of different proposals put forward for achieving this. In the field of residual current devices, it is customary to detect an imbalance in current flow through live and neutral conductors by passing the conductors through a toroid of magnetic material and having a sensing winding around part of the toroid. This arrangement is such that if there is an earth leakage fault, there is an imbalance between the current flowing in the live conductor with respect of the current flowing in the neutral conductor, and this imbalance results in a current being induced into the sensing winding. This winding is then connected to suitable circuitry for indicating the presence of the fault condition.
Residual current devices have become more and more popular and there is consequently a continuing need to be able to manufacture such devices in as efficient and cost effective a manner as possible. Attention has therefore been directed to the need for a sensor coil to be wound on a toroid as this is a relatively expensive operation.
European Patent Application EP-A-0242560 describes an arrangement for non-contact measurement of the electric current in a conductor, and relates to a device having a magnetic core surrounding the conductor. In a gap in the magnetic core is a gap in which is positioned a magnetic field sensor. The aim of this prior invention is to increase the measuring sensitivity of the device, and this is achieved preferably by utilising a magnetic core whose cross-sectional area becomes greater with increasing distance from the gap.
It is known from the article "Stromsensor als Mikrosystem" (Elektronik, vol. 42 No. 5 October 1993. Munchen, DE; pages 42-44, XP 000396422; Ruhl et al) that different types of devices may be placed in the air gap of a toroidailly-shaped member in order to sense currents.